Inorg. Chem. 1994,33,5785-5793
5785
Macrobicyclic Tris(catecho1ate ligand) Complexes: Spectroscopy, Electrochemistry, and the Structure of K2[(H2-bicappedTRENCAM)MoO2] Markus Albrecht, Sonya J. Franklin, and Kenneth N. Raymond’ Department of Chemistry, University of California, Berkeley, California 94720 Received February 9, 1994@
The coordination chemistry of the macrobicyclic triscatecholate ligands bicappedTRENCAM (2), bicappedTPTTRENCAM (3), and bicappedTPTCAM (4). as well as the new ligand TPTCAM (5), has been investigated with Fe3+, V4+,Ti4+, and Ga3+. UV-visible spectroscopy suggests that the Fe(II1) and the V(1V) compounds possess pseudooctahedral geometry at the central metal atom. The electrochemistry of the vanadium complexes of the above ligands and some comparison compounds ([VbicappedTRENCAM)I2- (19),[V(ETA)Iz- (20)) has been probed by cyclic voltammetry. An exemplary 4d metal complex, [M00z(H2-bicappedTRENCAM)]~(U), has been prepared. As shown by the X-ray structure analysis of the dipotassium salt, this cis-MoOz complex is formed by coordination of the cation external to the macrobicyclic ligand. Crystal data for MoK~C36H38N8014~ DMF5HzO: space group P24c (No. 14), a = 13.322(2) A, b = 14.252(2) A, c = 27.885(3) A, p = 103,11(1)0, V = 5.157(3) A3, and Z = 4.
Introduction Because the tris(catecho1) siderophorel.z enterobactin is such a good ligand for highly charged metal cations such as iron(111) (log Kf= 49),3 several analogs which model its coordination behavior have been proposed and Although no siderophores are known which have a macrobicyclic structure, an extension of this class of analogs is the macrobicyclic catecholate compounds, in which the metal binding cavity is formed by two capping backbones and three catechol g r o ~ p s ? ~ In ~ .the ~ ~uncoordinated ,~~ ligands the catechol units might be expected to be highly predisposed toward metal binding.11J4 Varying the ligand backbones, and therefore the cavity size, allows for the systematic study of the metal binding behavior and coordination geometry. Recent investigations in these laboratories showed that several 3d metal complexes (Ti4+, V4+, Fe3+) of the ligand H6-bicappedTRENCAM (2)exhibit unusual distorted trigonal prismatic geometry at the metal center,lZJ3whereas main group metal complexes (A13+, Ga3+) prefer octahedral coordination geometry. l3 This behavior was attributed to the influence of n-bonding involving the partially filled 3d shells of the metal ions. The iron(1II) complex of the
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, November 1, 1994. (1) Matzanke, B. F.; Muller-Matzanke, G.; Raymond, K. N. In Iron Carriers and Iron Proteins; Loehr, T. M., Ed.; VCH Publishers: New York, 1989; p 1. (2) Raymond, K. N. Coord. Chem. Rev. 1990,105, 135. (3) Loomis, L. D.; Raymond, K. N. Inorg. Chem. 1991,30, 906. (4) Kiggen, W.; Vogtle, F. Angew. Chem., Int. Ed. Engl. 1984,23, 714. ( 5 ) Stutte, P.; Kiggen, W.: Vogtle F. Tetrahedron 1987,43, 2065. (6) Coleman, A. W.;Ling, C.-k;Miocque, M. Angew. Chem., Int. Ed. En,gI. 1992,31, 1381. (7) Tsd, B.; Kishi, Y. J. Am. Chem. SOC. 1993,115, 7893. (8) Garrett, T. M.; Miller, P. W.; Raymond, K. N. Inorg. Chem. 1989, 28, 128. 191 Weitl. F. L.:Ravmond. K. N. J. Am. Chem. SOC. 1979. 101. 2728. (10) Rodgers, S. J.; &e, C.-W.; Ng,C. Y.; Raymond, K. N. Inorg. Chem. 1987,26, 1622. (11) Stack, T. D. P.; Hou, Z.; Raymond, K. N. J. Am. Chem. SOC. 1993, 115, 6466. (12) Garrett, T.M.; McMuny, T. J.; Hosseini, M. W.; Reyes, Z. E.; Hahn, F. E.:Ravmond. K. N. J. Am. Chem. SOC. 1991. 113. 2965. (13) Karpishii, T. B.;Stack, T. D. P.; Raymond, K. N. J. Am. Chem. SOC. 1993,115, 182. (14) Shanzer, A.; Libman, J.: Lifson, S.; Felder, C. E. J. Am. Chem. SOC. 1986,108, 7609.
larger H6-bicappedTPTCAM (4), however, exhibits the more usual octahedral geometry,13 in contrast to the other 3d metal complexes of 2. The purpose of the present study is to investigate the coordination behavior of transition metals with the macrobicyclic ligands 2-4. These ligands show a stepwise increase in the available metal-binding cavity size with successive variation of the linking groups capping the three 2,3-dihydroxy terephthalamide units. This work focuses on the syntheses of metal complexes of 2-4 and the influence of ligand size on the electronic spectra and electrochemistry of the complex. The coordination chemistry of the new monocapped ligand H6and HZTPTCAM (3,and the bidentate ligands Hz-ETA BTA (6b)8 are presented for comparison. In contrast to the encapsulation of the relatively small, isotropic ions Ga3+,Ti4+, V4+,and Fe3+,the cage molecule 2 coordinates the larger, more complex cation Mo0zz+ external to the cavity. Compounds 1-6 are shown in Chart 1.
Experimental Section The metal complexes are stable when exposed to air; nevertheless the syntheses were performed under argon using degassed solvents. The ligand TREN (tris-(2-aminoethyl)amine,W. R. Grace and Co.) was distilled from sodium spheres; TIT (tris-(3-aminopropyl)amine) was prepared according to published procedure starting with tris(2cyanoethyl)amine;lS (2,2’,2”-Tris(2,3-dimethoxyterephthalic (2-mercaptothia~olinamide))triamido)triethylamine,~~ &-bicappedTRENC AM. 2HBr @),I3H6-bicappedTPTCAM.2HBr (3),13 H2-ETA (6a),16and H2BTA (6b),16 as well as Kz[V(bicappedTRENCAM)] (19),” and Kz[V(ETA),] (20)16 were prepared as described in the literature. All other compounds were reagent grade and were not further purified. ‘H NMR spectra were measured at 298 K on a Bruker AM-400 or AM-500 spectrometer. Absorption spectra were recorded on a HP 8450 W-visible spectrophotometer. IR spectra were recorded using a Nicolet 5DX FT IR spectrometer. Microanalyses were performed by the Analytical Services Laboratory, and FAB mass spectra by the Mass Spectrometry Laboratory, both of the College of Chemistry, University of Califomia, Berkeley. (15) Chin, J.; Banaszyzyk, M.; Jubian, V.; Zou X. J. Am. Chem. SOC.1989, 111, 186. (16) Dewey, T. M. Ph.D. Thesis, University of California,Berkeley, 1992. (17) Karpishin, T. B.; Dewey, T. M.; Raymond, K. N. J. Am. Chem. SOC. 1993,115, 1842.
0020-1669/94/1333-5785$04.50/0 0 1994 American Chemical Society
5786 Inorganic Chemistry, Vol. 33, No. 25, 1994
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Chart 1
&-bicappedTRENC AM
&-bicappedTPTTRENCAM
H6-bicappedTPTCAM
3
4
2
O +
R
g,
'"jTO
I""
no OH 0
HN R
R = E r HrETA 6a R = Bu: HyBTA 6b Electrochemistry of Vanadium Compounds. Cyclic voltammograms were collected using a BAS l00A electrochemical analyzer. All potentials were measured at a scan rate of 0.1 V/s under an argon atmosphere using a platinum electrode. Potentials are reported relative to the saturated calomel electrode (SCE). Concentrations of the vanadium compounds were about 0.1 mM in 0.1 M Bu4NPFdDMF. Under the same conditions, the Cp2Fe/CpzFef potential was measured at 0.50 V with a peak separation of 67 mV.I8 MG-BicappedTFTTRENCAM (8). A solution of (2,2',2"-tris(2,3dimethoxyterephthalic (2-mercaptothiazolinamide))triamido)triethylamine (2.033 g, 1.89 "01) in 500 mL of methylene chloride and a solution of TPT (tripropylenetetramine) (0.353 g, 1.94 "01) in 500 mL of methylene chloride (containing 1 mL of triethylamine and 10 mL of methanol) were added simultaneously (FMI pump; addition rate: 5 mL/h) to 3 L of vigorously stirring methylene chloride over the course of 5 days. After complete addition, the mixture was stirred ovemight and solvent evaporated to 300 mL. The solution was washed with 0.1 M aqueous NaOH, dried over MgS04 and solvent was removed to yield an oil which was purified by chromatography over silica (2-12% methanoymethylene chloride). The macrobicyclic compound was obtained as a white solid (1.002 g, 1.12 mmol, 59%). (18) Bard, A. J; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980.
'H NMR (CDCldTMS, 500 MHz): 6 7.99, 7.95 (2 m, 3 H each), 7.51, 7.41 ( 2 d, J = 6.3 Hz, 3 H each), 3.63, 3.57 (2 m, 6 H each), 3.49, 3.42 (2 s, 9 H each), 2.78, 2.65, 1.79 ppm (3 m, 6 H each). Hs-BicappedTPTTRENCAM-2HBr(3).12 To 8 (242 mg, 0.27 "01) in 30 mL of degassed methylene chloride at 0 "C, 2 mL of BBr3 were added. The yellow suspension which immediately formed was stirred for 4 days at ambient temperature. Slow addition of water (a total of 200 mL), evaporation of methylene chloride and subsequent heating of the aqueous solution at reflux for several hours resulted in a brown solution. At 4 "C a brown wax precipitated. The wax was repeatedly dissolved in hot methanol and taken to dryness until a cream colored solid was formed, which was then recrystallized from methanou ether to yield 3 as a gray-white solid (190 mg, 0.18 "01, 67%; 3 is air sensitive, when wet). 'H NMR (dmso-d6, 400 MHz): 6 12.09, 11.69 (2 s, 3 H each), 9.72, 9.46 (2 m, 1 H each), 8.82, 8.68 (2 m, 3 H each), 6.94, 6.75 (2 d, J = 8.5 Hz, 3 H each), 3.79, 3.61, 3.41, 3.22, 2.00 ppm (5 m, 6 H each). Anal. Calcd (found) for CssH5~Br~N~O12CH~0H.3H~0: C, 44.95 (45.16); H, 5.68 (5.53); N, 10.48 (10.39). Me6-TP"CAM (9). TPT (321 mg, 1.76 "01) and 2.3-dimethoxybenzoic mercaptothiazolide (1.50 g, 5.29 "01) were dissolved in 100 mL of CH2C12. The mixture was stirred for 5 h at which point no remaining mercaptothiazolide could be detected by tlc (silica, CH2C12: CHlOH = 955). The organic layer was washed 3 times with 0.1 M
Tris(catecho1ate ligand) Complexes
Inorganic Chemistry, Vol. 33, No. 25, 1994 5787
KOH, dried over MgS04 and taken to dryness. The remaining oil (one taken to dryness and the residue was purified by chromatography over spot on tlc) was used without further purification. Sephadex LH-20 (methanol) to yield 15 (41 mg, 0.039 "01, 80%). 'H NMR spectrum (400 MHz, dmso-d6): 6 9.58, 9.05 (2 d, 3 H 'H NMR spectrum (CDCls, 400 MHz): 6 8.02 (m, 3 H), 7.60 (dd, each), 7.06 (br., 6 H), 3.50, 3.38, 3.32, 2.92 , 1.71 ppm (5 m, each 6 J = 1.6, 8.0 Hz, 3 H), 7.11 (pseudo t, J = 8.0 Hz, 3 H), 7.00 (dd, J = 1.6, 8.0 Hz, 3 H), 3.87 (s, 18 H), 3.48, 2.54, 1.76 ppm (3 m, 6 H H). UVlvis spectrum (HzO, pH = 7): A,,, 399 nm ( E = 18000 M-' each). cm-l). Anal. Calcd (found) for C39&2K2N8012TiCH30H.4Hz0: C, 45.98 (45.61); H, 5.21 (5.18); N, 10.72 (10.26). &-TPTCAM.HBr (5).12 The oil 9 was dissolved in 50 mL of CHzK2[(bicappedTPTCAM)Ti](16). Compound 16 was synthesized Cl2 under argon and 3 mL of BBr3 were added. The yellow suspension from 4 via a method analogous to that for compound 15. A broad was stirred at ambient temperature for 3 days. Volatiles were removed single orange band was collected from the Sephadex column, which under vacuum. The residue was quenched with methanol, taken to after evaporation of the solvent gave 16 in 50% yield. dryness, and heated at reflux in water for 2 h. After cooling the solution 'H NMR spectrum (400 MHz, dmSO-d6): 6 8.92 (m, 6 H), 7.09 (s, to ambient temperature, the waxy precipitate was collected. This solid 6 H), 3.31 (m, 12 H), 2.81 (m, 12 H), 1.63 ppm (m, 12 H). UVlvis was resuspended in methanol or ether and solvent removed several times spectrum (H20, pH = 7): A,,,, 397 nm ( E = 17900 M-' cm-'). to yield the beige solid 5 (627 mg, 54%). Negative ion MS (NBA): mle = 905 ([HM-, 100 %), 943 (K[M]-, 'H NMR spectrum (400 MHz, dmSO-ds): 6 12.49 (s, 3 H), 9.20 (2 30%). Anal. Calcd (found) for C ~ Z & E K Z N ~ ~ I Z TC, ~ ' 47.82 ~HZ~: m, 4 H), 8.84 (m, 3 H), 7.24, 6.90, 6.67 (3 m, 3 H each), 3.34, 3.17, H,: 5.35 (5.08); N, 10.62 (10.30). 1.91 ppm (3 m, 6 H). Anal. Calcd (found) for C ~ O H ~ ~ B T N ~ O ~ ~ (48.01); .~H~O K,[(bicappedTPTTRENCAM)Ga](17). H6-bicappedTPlTRENC, 52.48 (52.86); H, 5.58 (6.0); N, 8.16 (7.79). was dissolved in methanol (30 CAM.2HBr (3) (57 mg, 0.058 "01) Ks[(bicappedTPTTRENCAM)Fe] (10). To H6-bicappedTPTmL) and 0.5 M KOWethanol(O.58 mL) was added. After addition of TRENCM2HBr (3) (62 mg, 0.063 m o l ) in 30 mL of methanol, Ga(acac)3 (19 mg, 0.057 "01) the mixture was stirred overnight. first 0.636 mL of 0.5 M KOWethanol (0.318 mmol), and then iron Evaporation of solvent and chromatography (Sephadex LH-20l (111) acetylacetonate (20 mg, 0.063 "01) were added. The dark red methanol) yielded 17 (53 mg, 85%). mixture was stirred for 1 h and solvent was subsequently removed under 'H NMR spectrum (400 MHz, dmso-d6): 6 11.05, 10.85 (2 m, 3 H vacuum. The residue was purified by chromatography (Sephadex LHeach), 6.66 (br., 6 H), 3.39, 3.31,2.93,2.33, 1.63 ppm (4 m, 6 H each). 20, methanol). After evaporation of the solvent, the dark red solid 10 Anal. Calcd (found) for C ~ ~ H ~ ~ K ~ N E O I ~C,G43.62 ~ ~ H(43.82); ZO: was obtained (58 mg, 0.054 "01, 86%). which was recrystallized H, 4.69 (4.66); N, 10.43 (10.11). from wet DMFlether. K~[(bicappedTPTCAM)Ga](IS). Compound 18 was synthesized IR (KBr): 2945, 1609, 1550, 1441, 1202 cm-I. UVlvis spectrum from 4 in 78% yield via the method described for compound 17. (HzO, pH = 7): A,,, 363 nm ( E = 8600 M-' cm-I), 444 nm ( E = 4900 'H NMR spectrum (dmSO-d6, 500 MHz): 6 10.68 (m, 6 H), 6.71 (s, M-' cm-I), 508 nm ( E = 4250 M-I cm-l). Negative ion FAB MS 6 H), 3.36, 2.82, 1.53 ppm (3 m, 12 H each). Anal. Calcd (found) for (NBA): mle = 948 (K2[M]-, 25%), 910 (K[HM]-, loo%), 872 C42H48K3N8012GaCH3OH.3Hz0: c,45.71 (45.36); H, 5.17 (4.89); N, ([H2M]-, 80%). Anal. Calcd (found) for C ~ ~ H ~ Z F ~ K ~ N ~ O I ~ . ~ . ~(H~C)Z9.92 (9.42). NCHO.3HzO: C, 44.81 (44.80); H, 4.78 (5.69); N, 11.61 (11.99). Na~[(Hz-bicappedTRENCAM)MoOz] (24). To &-bicappedTRENK3[Fe(TPTCAM)] (11). The Fe(II1) complex of 5 was made in an CAM2HBr (2) (60 mg, 0.064 mmol) in 40 mL of methanol under analogous fashion to the synthesis of 10, yielding the dark red complex argon, 0.64 mL of a 0.5 M KOWethanol solution (0.32 mmol) was 11. added. The yellow solution which immediately formed was added to UVlvis spectrum (HzO, pH = 7): A,,, 333 nm ( E = 14000 M-l molybdenyl acetylacetonate (21 mg, 0.064 mmol) in 20 mL of cm-I), 508 nm ( E = 3460 M-l cm-'). methanol, at which point the mixture turned orange. After one hour Kz[(bicappedTPTTRENCAM)V] (12). H6-bicappedTPTTRENat room temperature the mixture was acidified with 1 M aqueous HBr. C M 2 H B r (3) (60 mg, 0.061 mmol) was dissolved in 20 mL of At pH < 6 a grey-green solid precipitates, which is isolable at pH = methanol, to which 0.488 mL of 0.5 M KOWethanol (0.244 "01) 4, and was shown to be &-bicappedTRENCAM (2). Using 1 M were added. After addition of vanadyl acetylacetonate (16 mg, 0.060 NaOH (approximately 1 to 1.5 mL) and 1 M HBr, the pH of the mixture mmol) the solution initially turned green but gradually became dark was adjusted as low as possible without precipitation (about 6) to obtain blue. A drop of 1 M HBr was added and the mixture was heated at a clear orange solution. After evaporation of solvent the residue was reflux overnight. After evaporation of solvent the residue was applied to column (Sephadex LH-20, methanol) to obtain 24 as an chromatographed (Sephadex LH-20, methanol) to yield dark blue 12 orange solid (57 mg, 0.060 mmol,94%) which was recrystallized from (53 mg, 0.051 mmol, 84%). wet DMF/ether. UVlvis spectrum (H20, pH = 7): A,, 346 nm ( E = 10800 M-I 'H NMR spectrum (D20,400 MHz): 6 6.86 (s, 4 H), 5.25 (s, 2 H), cm-I), 450 nm, ( E = 6900 M-'cm-I), 584 nm ( E = 8550 M-I cm-I), 3.55, 3.26, 3.08, 2.89, 2.74, 2.54 ppm (6 m, 4 H each); (dmso-d6,400 644 nm ( E = 8450 M-' cm-I). Negative ion FAB MS (NBA): mle = MHz): 12.28 (s, 2 H), 9.27 (m, 4 H), 6.70 (s, 4 H), 6.33 (m, 2 H), 866 [HMI-. Anal. Calcd (found) for C ~ ~ ~ ~ K Z N & V C HHzO: ~OH~ 5.61 (s, 2 H), 3.33, 3.21, 2.70 ppm (3 m, 24 H). IR (KBr): 899, 863 C, 45.84 (46.08); H, 5.19 (5.19); N, 10.69 (10.45). cm-' (cis-MoOz). Negative ion FAB MS (TEA): mle = 905 K2[(bicappedTPTCAM)V](13). The vanadium complex of 4 was (H[HzM]-, 40%). W l v i s spectrum (H20, pH = 7): A,,,= 370 nm ( E synthesized in an analogous fashion to compound 12, yielding 13 as a = 10650 M-l cm-'). Anal. Calcd for C36H38MoN8Na2014'2(H3c)~dark blue solid (82%). NCH04HzO: C, 43.22 (43.13); H, 5.18 (5.32); N, 12.00 (11.94). W l v i s spectrum (H20, pH = 7): A,,,=350 nm ( E = 9400 M-'cm-I), Na~[(BTA)&io02] (25). The dihydrate of sodium molybdate (50 448 nm ( E = 4700 M-'cm-'), 588 nm ( E = 6330 M-'cm-I). Negative mg, 0.21 "01) was suspended in 5 mL of ethanol under argon. To ion MS (NBA): mle = 908 [HMI-. Anal. Calcd (found) for this suspension a solution of Hz-BTA (6b) (199 mg, 0.65 "01) in 15 C42&8KzN8012V*5H2O: c , 46.88 (46.37); H, 5.43 (5.01); N, 10.41 mL of ethanol was added. The solution turned yellow-green and was (10.1 1). stirred at ambient temperature for 2 h. After an additional two hours Kz[(TPTCAM)V] (14). The vanadium complex of 5 was syntheat reflux the solution became brown. The solvent was evaporated and sized as described for compound 12, although the addition of HBr was the residue was applied to a colunin (silica; 5-20% methanoVmethylene omitted prior to heating at reflux. The dark blue product 14 was chloride). After evaporation of solvent an orange solid was obtained obtained in 91% yield. (130 mg, 0.17 "01, 81%). which was recrystallized from wet DMFl UVlvis spectrum (HzO, pH = 7) : A, 322, 440, 585 nm. Anal. ether. Calcd (found) for C30H30K2N409V'2H20: C, 47.68 (47.40); H, 4.53 'H NMR spectrum (dmso-d6, 400 MHz): 6 9.19 (m, 4 H), 6.95 (s, (4.42); N, 7.41 (7.30). 4 H),3.12 (m, 8 H), 1.20 (m. 16 H), 0.78 ppm (t, J = 7.1 Hz, 12 H). K2[(bicappedTPTTRENCAM)Ti] (15). To H6-bicappedTPTIR (KBr): 899,864 cm-l (cis-MoOz). Negative ion FAB MS (NBA): TRENCAMa2HBr (3) (48 mg, 0.049 "01) in 30 mL of methanol, mle = 765 (Na[M]-; 85%). Anal. Calcd (found) for C~ZHMMON~first 0.392 mL of 0.5 M KOWethanol, and then (acac)2TiO (13 mg, N~~OIO.~(H~C)~NCHO*~SH~O: C, 47.55 (47.53); H, 6.41 (6.04); N, 0.050 "01) were added. After stirring overnight, the mixture was 8.76 (8.87).
5788 Inorganic Chemistry, Vol. 33, No. 25, 1994
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Table 1. 'H NMR Shifts of the Ligands 2, 3, and 4 (.2HBr), and the Ti (15, 16), Ga (17, 18), and Mo (24) Complexes in dmso-ds 2.2HBr 42HBr 4.2HBr 15 16 17 18 24 OH 11.75 (s) N+H 10.05 (s) NHamide 8.93 (m) CH,i CHakyi
12.09, 11.69 (2s) 9.72, 9.46 (2s) 8.82, 8.68 (2m)
12.82 (s) 9.81 (s) 8.87 (m)
12.28 (s) 9.58, 9.05 (2m)
8.92 (m)
11.05, 10.85 (2m)
10.68 (m)
9.27 (br, 4H)*, 6.33 (br, 2H) 7.06 (br) 7.09 (s) 6.66 (br) 6.71 (s) 6.70 (s, 4H)*, 7.23 (s) 6.78 (s) 6.94, 6.75 (2d) ) 5.61 (s, 2H) J = 8.5 Hz) 3.80, 3.73 3.79, 3.61, 3.41, 3.47,2.99, 3.50, 3.38, 3.32, 3.31, 2.81, 3.39, 3.31, 2.93, 3.36, 2.82, 3.33, 3.21, (2m) 3.22, 2.00 (5m) 1.94 (3m) 2.92, 1.71 (5m) 1.63 (3m) 2.33, 1.63 (5m) 1.53 (3m) 2.70 (3m)
* Coordinating catechol units! Scheme 1
high dilution 5 - 7.5 days
\
7
0
NH2
/ 3
\
,./- - -
X-ray Diffraction. Bright orange plate-like crystals of compound 24' were obtained from slow diffusion of ether into wet DMF from which a fragment was cut and mounted in Paratone. Data collection and cell parameters are summarized in Table 4. Intensity standards measured every 1 h of exposure time indicated no significant decomposition occurred during data collection. The same three reflections were checked against their predicted positions every 200 reflections and crystal orientation was redetermined if any of the three deviated by more then 0.1". The crystal was reoriented once during the 36.2 h of exposure. The structure was solved by Patterson methods. Refinement was performed on F, with a dampening factor of 0.7. Ligand hydrogen positions were predicted and included in the structure factor calculation, but not refiied. The structure showed extreme disorder in the solvent and potassium counterions. Peaks attributed to DMF and water ran in well-defined channels approximately along the b axis. The disorder was modeled as two DMF molecules of 0.5 multiplicity and as nine oxygen positions with a total multiplicity of 5.35 oxygens. The thermal parameters of each of two groups of potassium ion fragments were constrained to be equal and the multiplicities of five positions refined to a total multiplicity of 1.983 potassium atoms. All partial occupancy atoms were refined isotropically. Prior to the final refinement, 16 reflections with abnormally large residual values were weighted to zero. The final residuals for 701 variables refined against 3272 data were R = 6.34 % and R , = 7.23%, with GOF = 1.634.
r
Results and Discussion Syntheses. The symmetric macrobicyclic ligands H6bicappedTRENCM2HBr (2)and H6-bicappedmcM2I-IBr (4)were synthesized via the method13 outlined in Scheme 1. The yields of the high dilution macrocyclizations (52%(2),64% (4))are extraordinarily high for this type of reaction. The unsymmetric ligand I-k-bicappedTPTTRENCM2HBr (3) containing one TREN and one TFT backbone was obtained in a similar fashion, with a 59% yield for the macrocyclization step. Metal complexes were obtained by the addition of the ligands 2-4 to appropriate metal compounds in methanol, followed by chromatographic workup (Sephadex LH-20, methanol). *HNMR Spectroscopy. The 'H-NMR spectra of the ligands 2-4 (QHBr) in dmSO-& show the high symmetry of the molecules (see Table 1). Compounds 2 and 4 each possess 3-fold symmetry (axis through the apical nitrogen atoms) and a mirror plane dividing the three catechol units. Ligand 3, however, lacks this mirror symmetry because of its two different backbones, leading to the duplication of the 2,3-dihydroxy terephthalamide signals relative to 2 and 4.19 In contrast to the paramagnetic Fe(II1) and V(1V) complexes,20-21 the Ti(1V) and Ga(II1) c ~ m p l e x e can s ~ be ~ ~studied ~~
Inorganic Chemistry, Vol. 33, No. 25, 1994 5789
Tris(catecho1ate ligand) Complexes
Scheme 2
KOH (acachM=O or (acac)3M m K*
* 2HBr
n = 1,2: 3,4
M = Ti'" (15,16),VIv (12,13) m = 2 M = Fem (lo),Gam (17,18) m = 3 "inside" coordination
in detail by NMR; 'H NMR shifts of the metal complexes 15 and 16 (Ti(1V)) and 17 and 18 (Ga(II1)) in dmso-d6, along with the shifts of the uncoordinated ligands 2-4 (-2HBr), are shown in Table 1. As observed in the ligand spectra, the compounds 16 and 18 show only one set of signals for the three catechol bisamide units, due to the 3-fold symmetry. In addition, mirror or inversion symmetry leads to equivalence of, for example, the amide protons of the terephthalamide units. Although they contain 3-fold symmetry, this further equivalence is not observed for the bicappedTPTTREN complexes (15, 17), where two different amide signals are detected. A further remarkable feature of the complex spectra is the downfield shift of the amide protons (up to 11.05 ppm in 17) compared to the corresponding protons in the free ligands. This suggests hydrogen bridging between the amide nitrogen and the catecholate oxygen atoms that coordinate to the metal centers,24 similar to that seen in other 2,3-dihydroxy terephthalamide c ~ m p l e x e s . ~Although ~ ' ~ ~ the ~ ~'H~ NMR ~ ~ ~spectra ~ ~ are consistent with a highly symmetric environment in compounds 1518, no distinction between trigonal prismatic and octahedral geometry at the metal center can be made based on this spectroscopic method. UV-Visible Spectroscopy. Recently it was shown that W-visible spectroscopy of 2,3-dihydroxyterephthalamidecomIn solution with low concentration of H6-bicappedTRENCAM.2HBr (2), only the mono-HBr salt can be observed which shows no mirror plane perpendicular to the terephthal units (compare with ref 13). Only a high concentration of 2 allows the detection of the bis-HBr adduct. See for [F~(OZC&)~]~-: Raymond, K. N.; Isied, S. S.; Brown, L. D.; Fronczek, F. R.; Nibert, J. H. J. Am. Chem. Soc. 1976, 98, 1767. See for [v(02C6H4)3]2-: Cooper, S. R.; Koh, Y. B.; Raymond, K. N. J. Am. Chem. SOC. 1982, 104, 5092. See for [T~(O~CFJ&)~]~-: Borgias, B. A,; Cooper, S. R.; Koh, Y. B.; Raymond, K. N. Inorg. Chem. 1984, 23, 1009. See for [Ga(o~C6&)3]~-:Borgias, B. A,; Barclay, S. J.; Raymond, K. N. J. Coord. Chem. 1986, 15, 109. Hesse, M.; Meier, H.; Zeeh, B. Spektroskopische Methoden in der organischen Chemie; Georg Thieme Verlag: Stuttgart, New York, 1984; p 172. Bulls, A. R.; Pippin, C. G.; Hahn, F. E.; Raymond, K. N. J. Am. Chem. SOC. 1990. 112. 2627. Karpishin, T. B.; Stack, T. D. P.; Raymond, K. N. J. Am. Chem. SOC. 1993, 115, 6115.
plexes gives some indication of the geometry at the central Because of increased catecholate oxygen atom to metal n-interaction in trigonal prismatic complexes, the change from an octahedral to a trigonal prismatic geometry has a dramatic influence on the energy and the intensity of the ligand to metal charge transfer (LMCT) transition^.'^^'^^^^ Although the spectra of Ga3+ (d'O), and Ti4+ (do) complexes (maximum at il= 399, 15; 397 nm, 16) are not particularly useful, the Fe3+ (10, 11) and V4+ compounds (12-14) have informative spectra. The absorption spectra of [Fe(bicappedTPTTRENCAM)I3-(lo), [Fe(TPTCAM)I3- (ll),[V(bicappedTRENCAM)I2- (19), [V(bicappedTFTTRENCAM)I2- (12), [V(bicappedTPTCAM)I2(13), and [V(ETA)3I2- (20) in aqueous solution at pH = 7 are presented in Figure 1. In the iron(II1) complexes 10 and 11, the bands at about 363 nm and 335 nm, respectively, arise from ligand (n-n*) transitions. The two transitions at lower energy are assigned to ligand to metal charge transfer (LMCT) transition^.^^^^^ Previous work has shown that the transition at about 510 nm in these systems varies only slightly in changing from octahedral to trigonal prismatic geometry.27 However, the higher energy LMCT band is somewhat indicative of the geometry around iron. In the spectrum of [Fe(bicappedTPTTRENCAM)l3-(lo), the band at 444 nm ( E = 4900) is very typical for pseudooctahedrally coordinated iron(II1) tris(2,3-dihydroxyterephthalamide) compound^.'^ In the trigonal prismatic compound [Fe(bicappedTRENCAM)I3- (21) this transition is observed at higher energy, with a larger extinction coefficient (428 nm, E = 7200 M-' cm-') due to a larger degree of n bonding.13 The spectrum of [Fe(TPTCAM)l3- (11) with a LMCT transition at (27) Karpishin, T. B.; Gebhard, M. S.; Solomon, E. I.; Raymond, K. N. J. Am. Chem. Soc. 1991, 113, 2977. (28) Garrett, T. M. Ph.D. Thesis, University of California, Berkeley, 1988. (29) Garrett, T. M.; Cass, M. E.; Raymond, K. N. J. Coord. Chem. 1992, 25, 241. (30) For rare examples of homoleptic molybdenum catechol complexes see: (a) Pierpont, C. G.; Buchanan, R. M. J. Am. Chem. SOC.1975, 97, 4912. (b) Cass, M. E.; Pierpont, C. G. Inorg. Chem. 1986, 25, 123. (c) Pierpont, C. G.; Downs, H. H. J. Am. Chem. Soc. 1975, 97, 2123.
Inorganic Chemistry, Vol. 33, No. 25, 1994 15000 12500
,
I
-
,
1
I
Albrecht et al. 1
-
[Fe(TPTCAM)I"
-
Table 2. Redox Potential, Peak Separation, and Current Ratio Observed for the Vanadium Compounds 12-14, 19, and 20 (DMF vs SCE)"
[Fc(bicappedTPTTRENCAM)I'* .--.--
10000
-
-
7500
-
-
(a)
300
400
500
600
700
I
I
I
I
Vvnv (V) [V(TPTCAM)Iz-(14) +0.28 [V(bicappedTRENCAM)]'- (19) f0.67 [ V ( ~ ~ C ~ ~ ~ ~ ~ T P T T R E N+0.64 CAM)]~(12) [V(bi~appedTPTCAM)1~(13) +0.66 [+0.99 (ox.)] [V(ET&l2- (20) f0.72 (ox.)
Peak
separation current (mV) ratio 71 78 88
1.18 1.10 1.24
95
2.16
+0.90 (ox.) 4-0.15 (red.)
800
wavelength [nml
12000
redox potential
a Conditions: ca. 0.1 mM metal complex concentration, 0.1 M BmNPF6, Pt working electrode, 0.1 Vls.
I
I
1
Scheme 3
10000
8000 6000 4000 2000 2
24
"outside"coordination
0
200
300
400
500
600
700
800
900
(current ratio = 2.16) and a second oxidation step at f 0 . 9 9 V. Similar redox properties can be detected for [V(ETA)#- (20) Figure 1. UV-visible spectra of (a) 10 and 11, and (b) 19, 12, 13, (oxidations at f 0 . 7 2 and +0.90 V, reduction at +0.15 V). This and 20. irreversible behavior may be due to dissociation of one of the 494 nm is very similar to that described for the octahedral three ligands. The increased potential of the bisamide ligands compared to monoamide ligands such as 14 shows that these complex [Fe(mENCAM)l3- (22).1° ligand systems stabilize the V(1V) oxidation state relative to The spectra of the vanadium compounds presented here have been assigned by analogy to the four characterized transitions V(V). This is consistent with the less basic character of the observed for [V(enterobactin)12- (23).17 These compounds bisamide catecholate oxygen atoms compared to the monoamide ligands, and appears to lead to dissociation of the V(V) exhibit a ligand (n-n*) transition at about 350 nm, and LMCT compounds with flexible ligands like 4 or 6. transitions at lower energy (between 400 and 800 nm). In the green trigonal prismatic complex [V(bicappedTRENCAM)I2The quasi-reversible redox behavior of compounds 14, 19, (19), two absorptions with high extinction coefficients are and 12, in contrast to the behavior of 13 and 20, is also reflected observed at 450 and 630 nm (c = 10500 and 9400 M-km-', in the air sensitivity of water solutions of the complexes. Dark respectively). The transitions of the dark blue octahedral blue solutions of 13 or 20 decolorize within a few days (7 days complex [V(ETA)3I2- (20),16on the other hand, show lower or 1 day, respectively), whereas solutions of 12,14, or 19 show extinction coefficients, especially for the transition at about 440 no change in color over a period of several weeks. nm; [V(bicappedTPTTRENCAM)I2(12), [V(bicappedTPTCSpectroscopic Features of 24. A spectroscopic analysis of AM)I2- (13), and [V(TPTCAM)IZ- (14) (not shown) have [(H~-bicappedTRENCAM)Mo0~]~(24)shows the Moo2 unit spectra and color very similar to that of [V(ETA)& (20), has the usual cis geometry. The typical vibrational frequencies indicating that those compounds also are essentially octahedral for a cis-Moo2 unit are found in the IR spectrum of 24 at 899 at the metal center. and 863 cm-'; Na2[(BTA)2Mo02] (25) exhibits nearly the same Voltammetric Measurements of the V(1V) Compounds. Mo-0 bands at 899 and 864 cm-1.31 Cyclicvoltammetric investigations of iron(II1) complexes with Two different sets of signals for the ligand 2,3-dihydroxy the ligands 2-4 are described elsewhere.12 The electrochemistry terephthalamide moieties are observed in the 'H NMR spectrum of the complexes [V(ETA)#- (20), [V(TPTCAM)I2- (14), [Vof 24 in dmso-ds (Figure 3, Table 1). The signals occur in a (bicappedTRENCAM)I2- (19), [ V ( ~ ~ C ~ ~ ~ ~ ~ T F T T R E N C 2: A 1Mintegration ) ] ~ - ratio. The difference in the two sets' aromatic (12), and [V(bicappedTPTCAM)I2- (13) were investigated in and amide proton chemical shifts, as well as the shifts of the 0.1 M B d P F f l M F containing about 0.1 mmol of metal two hydroxo group protons (2H at 12.28 ppm), indicate the complex. The cyclic voltammograms in which the waves are presence of two coordinated and one uncoordinated, protonated assigned to the VVavredox potential are shown in Figure 2 and catecholate unit. The catechol H,1(6.70 ppm) and Hamide (9.27 Table 2. A quasi-reversible wave at +0.28 V is observed for ppm) protons of the larger set occur in the shift regions typical [V(TPTCAM)I2- (14),which is typical for 2,3-dihydroxy benzfor coordinated catechol groups, and are very similar to the shifts amide ~omp1exes.l~ The waves of [V(bicappedTRENCAM)I2seen in the spectrum of 25 (6.95 (H,') and 9.19 ppm (Hhde)). (19) (+ 0.67 V) and [V(bicappedTPTTRENCAM)I2-(12) (+ 0.64 V) are essentially reversible, whereas [V(bicapped(31) Griffith, W. P.;Pumphrey, C.A,;Rainey, T.-A. J. Chem. Soc., Dalton TPTCAM)I2- (13) shows an irreversible potential of +0.66 V Trans. 1986, 1125. wavelength [nm]
Inorganic Chemistry, Vol. 33, No. 25, 1994 5791
Tris(catecho1ate ligand) Complexes
+” 1300
[V(bicappedTRENCAM)jZ-
t 1,
1
7
7
3
0
0
[V(bicappedTPTCAM)I2
Figure 2. Cyclic voltammograms of the vanadium complexes 14, 19, 12, 13, and 20 in DMF vs SCE. Conditions: ca. 0.1 mM metal complex concentration, 0.1 M BwNPF6, Pt working electrode, 0.1 V/s.
CH
-,. . . . . , . . . . . . . . , . . . . 12.0
10.0
”..,.”.r“
0.0
6.0
PPM
Figure 3. Section of the ‘HN M R spectrumof 24 in dmso-d6presenting the signals of the aromatic, amide, and hydroxy protons.
The H d d e shifts are in the region for amide protons forming hydrogen bridges. For the uncoordinated terephthalamide of 24, upfield shifts occur because of the anisotropic effect of the two coordinated terephthalamides aromatic rings influencing the shielding of the protons situated between those two rings. The amide protons appear at 6.33 ppm and the aromatic protons at 5.61 ppm (5.25 ppm in DzO!). This is evidence that the aromatic and the amide protons of the uncoordinated catecholamide point into the
interior of the cavity of the macrobicyclic molecule. The hydroxo functionalities and the amide oxygen atoms, however, are directed away from the macrocycle, and the cis-Moo2 unit is coordinated to the “outside” of the cage molecule. X-ray Crystal Structure of Kz[(Hz-bicappedTRENCAM)MoOz] (24‘). X-ray quality crystals of 24’ were obtained by diffusion of ether into wet DMF (approximately 5- 10% water). Although elemental analysis shows that the bulk sample 24 was obtained as the disodium salt, X-ray diffraction established that the cation in the crystal selected from the bulk sample was potassium, introduced in the initial step of the synthesis in which KOH was used for deprotonation. The potassium salt 24’ crystallizes in the monoclinic space group P21/c (No. 14) with cell parameters described in Table 4. The X-ray structure nicely elucidates the general features of the dianion [(H~-bicappedTRENCAM)Mo02]~(Figure 4, Table 3) although the cations and the various solvent molecules (five water, one DMF) show some disorder. The Moo6 unit possesses a distorted octahedral geometry with the two double bonded oxygen atoms oriented cis to each other (d(Mo=O) = 1.722(7), and 1.709(8) A; O=Mo=O = 99.1(4)’). This cisMoo2 unit is coordinated “outside” of the cavity of the macrobicyclic molecule as was expected based on spectroscopic data. One of the two oxygen atoms of this unit is within hydrogen bonding distance of a water molecule ( d ( O . 0 ) = 2.856 A).32 Further hydrogen bonding seems to occur between the amide and catecholate functionalities of the coordinated units. The observed geometry is typical for dicatechol molyb-
Albrecht et al.
5792 Inorganic Chemistry, Vol. 33, No. 25, 1994
Figure 4. Molecular structure of the dianion [(H2-bicappedTRENCAM)Mo02I2-. 9 C 2 9
Table 4. Summary of Crystal Data for K2( H2-bicappedTRENCAM)MoOz] (24') formula: C39Hs3MoN9020 1142.01 fw -90 temp ("C) triclinic crystal system P ~ ~ / c ( N 14) o. space group cell constso 13.322(2) a (A) 14.252(2) b (A) 27.885(3) c (A) 90 a (deg) 103.11(1) P (de& 90 Y (de& 4 z 5156.7(2.6) v (A3> 4.86 abs coeff, pcalc (cm-') 1.47 1 & I C 2360 F(ow 0.25 x 0.25 x 0.10 mm cryst dimens Mo K a (A = 0.71073 A) radiation Enraf-Nonius CAD-4 diffractometer 0 14, -15 15, -30 h,k,Z range collctd 3.0-45.0" 28 range 8-28 scan type 5.49 scan speed (e, deg/min) 8492 no. of reflcns collcd 6743 no. of unique reflcns 3272 no. of reflcns with (Fo2 > 3*a(F;)) 701 no. of paramrs 4.7 datdparam ratio 0.064 R = [CIAFI/CIFoll 0.072 R, = [Cw(Afl2/CwFO2] 1.634 GOF +0.629'/-0.165" final diff @"+/emax(e-/A3)
Wo7
-.
-.
-30
::"& 08
Figure 5. Presentation of (a) the two coordinating, and (b) the noncoordinating 2,3-dihydroxy terephthal amide units of 24. Table 3. Selected Bond Lengths (A) and Angles (deg) for 24' Mo-01 Mo-02 Mo-03 Mo-04 Mo-05 Mo-06
1.722(7) 1.709(8) 2.176(7) 1.997(8) 2.167(7) 1.991(7)
0 1-Mo-02
99.14(4)
03-Mo-04 05-Mo-06
(74.3(3) 74.8(3)
denum dioxo d i a n i ~ n s . ~ The ~ - ~ uncoordinated ~ dihydroxyterephthalediamide moiety of 24' is oriented parallel to one of the coordinated units (nstacking) with the aromatic and the amide protons pointing towards the n system of the third aromatic group. This is evidenced in solution by the anisotropic shift in the 'H NMR spectrum. Compound 24' gives the opportunity to compare coordinated and uncoordinated 2,3-dihydroxyterephthalamideunits in one molecule. Both of these units are nearly planar, with slight distortions due to the strain of the macrobicyclic molecule. Figure 5a presents the two coordinating moieties. The nitrogen atoms of the amide groups are oriented in the same direction as the catecholate oxygen atoms, allowing hydrogen bonding between these two functionalities (d(0-N) = 2.699, 2.702, or 2.708, 2.732 A). In contrast to this conformation, the uncoordinated unit is rotated approximately 180" about the CamideCmomatic bonds (Figure 5b). This leads to an orientation of the four oxygen atoms (two catechol, two amide) in the same See for a comparison: Haselhorst, G.; Stotzel, S.; Strassburger, A.; Walz, W.; Wieghardt, K.; Nuber, B. J. Chem. SOC.Dalton Trans. 1993, 83. Yamanouchi, K.; Enemark, J. H. in Third International Conference on Molybdenum Chemistry; Barry, H. F., Mitchell, P. C. H., Eds.; Climax: New York, 1979; p 24. Pierpont, C. G.; Buchanan, R. M. Coord. Chem. Rev. 1981, 38, 45 and literature cited therein. Dowerah, D.; Spence, J. T.; Singh, R.; Wedd, A. G.; Wilson, G. L.; Farchione, F.; Enemark, J. H.; Kristofzski, J.; Bruck, M. J. Am. Chem. SOC. 1987, 109, 5655.
a Unit cell parameters and their esd's were derived by a least-squares fitting of the setting angles of 24 reflections in the range 23.728" I28 5 24.610". Located near K2 fragments. Located near K2 fragments. An empirical absorption correction (EAC) was applied based on azimuthal scans: min(av) = 86.1%.
direction, allowing hydrogen bridging between catefhol and amide oxygen atoms ( d ( 0 . 0 ) = 2.551 and 2.510 A). This conformational difference between coordinated and uncoordinated catecholamideligands has been observed earlier in X-ray studies of free ligands and their metal complexes, even those not constrained by macrocyclic structures. 12~13726729
Conclusions Macrobicyclic triscatecholate siderophore analogs are versatile in their ability to bind a variety of main group, 3d, and 4d metal ions. However, choice of backbones allows a fine tuning of the cage molecule's cavity size, which consequently influences metal coordination geometry. As shown earlier, the smallest ligand bicappedTRENCAM (2) is capable of enforcing an unusual trigonal-prismatic geometry at 3d metal centers.l2>l3 This is due to very tight ligand to metal interactions overcoming the ligand-ligand repulsion which is relatively larger in trigonalprismatic geometry than pseudo-octahedral geometry. As indicated by UV-visible spectroscopy, the somewhat larger bicappedTPTTRENCAM (3), as well as the largest ligand investigated here, bicappedTPTCAM (4), form pseudo-octahedral coordination compounds with Fe3+ or V4+. Cyclic voltammetry (CV) investigations of the vanadium compounds have been presented and compared to the vanadium complexes of the new ligand TPTCAM (5), and of ETA (6a). The CV measurements show that the terephthalamide ligands 2-4, and 6a are relatively better at stabilizing vanadium in the 44- oxidation state than analogous benzamide derivatives like TPTCAM (5). The macrobicyclic ligand cavity size is found to have a great influence on the reversibility of the oxidation
Tris(catecho1ate ligand) Complexes of the vanadium(1V) compounds. Unlike complex 4, which shows irreversible behavior very similar to that observed for the tris-ETA complex (20),the two smallest ligands (2,3) show reversible CV waves. This indicates the relatively tighter encapsulation of the oxidized cation prevents further reaction of the V5+ compounds. The terephthalamide units of bicappedTPTCAh4 (4) apparently act as three independent bidentate ligands, although they are connected by two large and flexible backbones. Unlike the other metal ions described here, the larger, nonspherical complex cation cis-Mo0Z2+ cannot be fully incorporated into the bicappedTRENCAM (2) cavity. Rather, the cation is coordinated to the “outside” of the macrobicyclic cage, as the crystal structure of 24’, which reveals the geometry of the dianion [(H2-bicappedTREiNCAM)MoO~]~-, nicely shows. The observed structure of 24‘ is the first where a cis-Moo2 unit is coordinated to a siderophore analog. Naturally occumng siderophores are well known to have an important part in the regulation of the uptake and transport of iron(III), and probably
Inorganic Chemistry, Vol. 33, No. 25, 1994 5793 of other biorelevant metals. In this context, structures like that of 24’have been proposed to be important regulators in the transport of molybdenum(V1) in organism^.^^,^'
Acknowledgment. We thank Dr. Frederick J. Hollander for his help with the crystallography. This research was supported by the National Institute of Health, Grants AI 11744 and, in part, DK 32999. M.A. gratefully acknowledges a NATO postdoctoral fellowship. Supplementary Material Available: X-ray crystallographic data for K2[(H2-bicappedTRENCAM)MoO2](24’), including tables of anisotropic thermal parameters, atom positional parameters, and nonessential inter- and intramolecular distances and angles (7 pages). Ordering information is given on any current masthead page. (36) Hider, R. C. Stmct. Bonding 1984,58, 26. (37) Wilshire, J. P.; Leon, L.; Bosseman, P.; Sawyer, D. T.; Buchanan, R. M.; Pierpont, C. G. In Third International Conference on Molybdenum Chemistry; Barry, H. F., Mitchell, P. C. H., Eds.; Climax: New York, 1979; p 264.
